A slave of an extension telephone system and a mobile telephone usually incorporate each a secondary battery that can be charged and discharged, and operate by receiving supply of power from the secondary battery. Thus, the slave and the mobile telephone need to be placed in a charger on a regular basis for charging the internal secondary battery.
For example, JP-A-9-215223 discloses that a switching element such as a transistor is placed between a secondary battery and a power supply section incorporated in a charger, the terminal voltage when the switching element is turned on/off is detected, and charging control is performed in response to the terminal voltage value, whereby the connection state of the secondary battery and the charger is detected.